Staged catalytic ammonia decomposition in integrated gasification combined cycle systems

ABSTRACT

The ammonia content of fuel gas in an IGCC power generation system is reduced through ammonia decomposition, thereby reducing the NO x  emissions from the plant. The power generation system includes a gasifier, a gas turbine and at least one catalytic reactor arranged between the gasifier and the gas turbine. The catalytic reactor may be either a three stage or two stage device. The three stage reactor includes a first catalyst which promotes water-gas-shift, a second catalyst which promotes CO methanation, and a third catalyst which promotes ammonia decomposition. The two stage reactor includes a first catalyst which promotes water-gas-shift and CO methanation and a second catalyst which promotes ammonia decomposition. The plural catalytic stages may be disposed in a single vessel or successively disposed in individual vessels, and the catalysts may be in a pelletized form or coated on honeycomb structures. Alternatively, fluidized bed reactors may be used. The reactions are carried out either adiabatically or non-adiabatically. Heat from the water-gas-shift and CO methanation reactions may be used to generate steam, which can be injected downstream or sent to a steam turbine. Preferably, a second catalytic reactor is provided in parallel with the first reactor so that the two reactors can alternately receive fuel gas from the gasifier.

This application is a division of application Ser. No. 08/269,797, filedJun. 30, 1994.

BACKGROUND OF INVENTION

This invention relates generally to a staged catalytic process forreducing the ammonia concentration in the gas produced by a gasifier,particularly an oxygen-blown coal gasifier in an integrated gasificationcombined cycle power plant equipped with high temperaturedesulfurization.

In integrated gasification combined cycle (IGCC) power plants, low Btufuel gas produced by a gasifier is burned and expanded through a gasturbine, and the exhaust heat from the gas turbine is used to generatesteam for a steam turbine. The low Btu fuel gas can be produced bygasifying coal, biomass, municipal solid waste, wood chips, heavyresidual oil, petroleum coke, refinery wastes and other materials. Asused herein, the term "fuel gas" refers to gas produced by any suchgasification process. IGCC systems are attractive because of their highefficiency and because they can use relatively abundant and/orinexpensive energy sources.

Since the fuel gas produced by gasification typically contains highlevels of hydrogen sulfide (H₂ S), a sulfur removal system must beemployed. Currently, both low temperature and high temperaturedesulfurization schemes are used. Hot gas clean up (HGCU) is a hightemperature sulfur removal scheme which has several advantages over lowtemperature schemes, most notably increased system efficiency anddecreased cost. HGCU reduces the sulfur in the fuel gas to less than 50ppmv H₂ S and is typically carried out in the range of approximately800-1200° F. This temperature regime is near optimal for desulfurizationbecause at temperatures below about 800° F. the overall power plantefficiency decreases, while at temperatures above about 1200° F. theefficiency and stability of the desulfurization sorbents decrease.However, high temperature fuel gas tends to have a high ammonia content,about 1000-2000 ppmv. This high ammonia content results in high NO_(x)emissions when the fuel gas is burned. Thus, the ammonia content of thehigh temperature fuel gas must be decreased to reduce NO_(x) emissions.

One way to reduce the ammonia content of the fuel gas is to promoteammonia decomposition. However, known catalysts that are active forammonia decomposition in the range of 800-1200° F. are easily poisonedby as low as a few parts per million of H₂ S. At temperatures wheresulfur poisoning is less of a problem (about 1400° F.), these catalystshave poor mechanical/chemical stability, i.e., loss of surface areabecause of sintering. Similarly, catalysts that are sulfur resistant andmechanically stable at 1400° F. tend not to be active enough towardsammonia decomposition at lower temperatures near 1000° F. Hence,operation of an ammonia decomposition catalyst at the same temperatureas a high temperature desulfurization system may not be easilyimplementable.

Accordingly, there is a need for a process and apparatus for reducingthe ammonia concentration of high temperature fuel gas which can fitinto the constraints of an IGCC power plant having high temperaturesulfur removal.

SUMMARY OF THE INVENTION

The above-mentioned needs are met by the present invention in which theammonia content of fuel gas is reduced through ammonia decomposition.This is accomplished with a staged catalytic process in which theammonia decomposition reaction is facilitated by first promoting COmethanation and water-gas-shift.

Specifically, the present invention provides a power generation systemcomprising a gasification unit, a hot gas desulfurization system, aparticulate removal system, and a gas turbine. A catalytic reactor isarranged between the hot gas desulfurization system and the gas turbine.The catalytic reactor contains a plurality of catalysts whichcollectively promote water-gas-shift, methanation of CO, and ammoniadecomposition. Preferably, a second catalytic reactor is provided inparallel with the first reactor so that the two reactors can alternatelyreceive fuel gas from the desulfurization system. A coolant injectionport or heat exchanger is formed in the gas line connecting said thecatalytic reactor and the gas turbine to cool the fuel gas.

Each catalytic reactor may be a three stage device that includes a firstcatalyst which promotes water-gas-shift, a second catalyst whichpromotes methanation of CO, and a third catalyst which promotes ammoniadecomposition. The catalytic reactor can comprise a single vesselcontaining all of the catalysts, or it can comprise a first vesselcontaining the first catalyst, a second vessel containing the secondcatalyst, and a third vessel containing the third catalyst. The threevessels are connected in order between the desulfurization system andthe gas turbine. The power generation system can further include heatexchangers disposed in the first and/or second vessels. The heatexchangers can be connected to the injection port or to the steamturbine.

Alternatively, the catalytic reactor may be a two stage device thatincludes a first catalyst which promotes water-gas-shift and methanationof CO, and a second catalyst which promotes ammonia decomposition. Thiscatalytic reactor can comprise a first vessel containing the firstcatalyst and a second vessel containing the second catalyst. The vesselsare connected in order between the desulfurization system and the gasturbine. If using heat exchange, a heat exchanger would be disposed inthe first vessel.

The vessels can be filled with catalysts in either a pelletized form ora fluidized bed. Alternatively, the catalytic reactor can comprise twoor three honeycomb structures. Each honeycomb structure would be coatedwith the appropriate catalyst.

An advantage of the staged catalytic operation of the present inventionis that it allows catalysts which individually are restricted to anarrow range of operating conditions to be used in sequence toaccomplish the goal of reducing ammonia content that is not feasible byany single stage.

Other objects and advantages of the present invention will becomeapparent upon reading the following detailed description and theappended claims with reference to the accompanying drawings.

DESCRIPTION OF THE DRAWINGS

The subject matter which is regarded as the invention is particularlypointed out and distinctly claimed in the concluding part of thespecification. The invention, however, may be best understood byreference to the following description taken in conjunction with theaccompanying drawing figures in which:

FIG. 1 is a schematic view of an IGCC system using staged catalyticammonia decomposition in accordance with the present invention;

FIG. 2 shows a first embodiment of a catalytic reactor of the presentinvention;

FIG. 3 shows a second embodiment of a catalytic reactor of the presentinvention;

FIG. 4 shows a honeycomb structure in accordance with the presentinvention;

FIG. 5 shows a third embodiment of a catalytic reactor of the presentinvention; and

FIG. 6 shows a fourth embodiment of a catalytic reactor of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention reduces the ammonia content of the fuel gasproduced by a gasifier by inducing ammonia decomposition. The minimumammonia content that can be achieved by ammonia decomposition isdetermined by the equilibrium ammonia concentration of the fuel gas;that is, the lower the equilibrium ammonia concentration of the fuel gasis, the more the ammonia content can be reduced. The equilibrium ammoniaconcentration is a function of the temperature and pressure of the fuelgas as well as the overall composition of the gas. Although theequilibrium ammonia concentration is usually lower than the ammoniacontent of fuel gas, conditions are such that ammonia decompositionalone is often insufficient to adequately reduce the ammonia content.Thus, the present invention facilitates ammonia decomposition bypromoting CO methanation and water-gas-shift.

The overall reaction that decomposes ammonia is

    2NH.sub.3 =N.sub.2 +3H.sub.2                               (1)

Since this reaction is endothermic in the forward direction andincreases hydrogen content, increasing the temperature of the fuel gasand reducing its hydrogen content will favor ammonia decomposition. Thepresent invention increases temperature and reduces hydrogen content bypromoting CO methanation and water-gas-shift. The CO methanationreaction is

    CO+3H.sub.2 =CH.sub.4 +H.sub.2 O                           (2)

and the water-gas-shift reaction is

    CO+H.sub.2 O=CO.sub.2 +H.sub.2                             (3)

Both of these reactions, which are typically not equilibrated at HGCUtemperatures (about 800-1200° F.), are exothermic in the temperaturerange of interest and thus raise the fuel gas temperature. Furthermore,the CO methanation reaction consumes H₂, thereby reducing the overallhydrogen content.

Referring to FIG. 1, an integrated gasification combined cycle (IGCC)system 10 of the present invention is shown schematically. The IGCCsystem 10 includes a gasification unit 12, a hot gas clean up (HGCU)system 14, a particulate removal system 15, and a gas turbine 16. As iswell understood in the art, the gasification unit 12 comprises agasifier, such as an oxygen-blown gasifier in which coal or othermaterials are reacted with steam in the presence of oxygen to producefuel gas which is ultimately burned and expanded in the gas turbine 16to generate power. In addition to a gasifier, the gasification unit 12typically includes a radiant cooler and a cyclone for removingparticulates. The fuel gas produced by the gasification unit 12 isdelivered to the HGCU system 14 which removes hydrogen sulfide from thefuel gas.

Two staged catalytic reactors 18, 20 are located between the HGCU system14 and the particulate removal system 15. Both of the catalytic reactors18, 20 contain a plurality of catalysts arranged in stages so as tocollectively promote methanation of CO, water-gas-shift, and ammoniadecomposition. Two or three catalytic stages are typically required. Inone embodiment, the staged catalytic reactors 18, 20 each comprise asingle vessel having an inlet and an outlet at opposing ends so thatfuel gas can be directed through each vessel. The plural stages ofcatalysts, in a pelletized form, are arranged in successive packed bedswithin the vessels.

The two catalytic reactors 18, 20 are connected in parallel so thatdesulfurized fuel gas from the HGCU system 14 can be caused. (viaconventional valves 22) to alternately pass through one or the other ofthe reactors 18, 20. In this way, only one of the reactors 18, 20 is inuse at any given time. Meanwhile, the idle reactor can be eitherregenerated or replaced. A coolant injection port 24 is provided in thegas line 26 between the catalytic reactors 18, 20 and the gas turbine16. The port 24 provides a means for the injection of a coolant, such aswater or low temperature nitrogen, into the gas stream, therebyquenching the fuel gas to an allowable temperature for gas turbineoperation (approximately 1000-1100° F.). A heat exchanger could be usedas an alternative to the injection port 24.

FIG. 2 shows another embodiment of the staged catalytic reactors 18, 20(for simplicity, only reactor 18 is shown) in which there is a separatevessel for each catalytic stage. The catalytic reactor 18 has threestages in the form of a first vessel 28 containing a first catalyst, asecond vessel 30 containing a second catalyst and a third vessel 32containing a third catalyst. The first catalyst promoteswater-gas-shift, the second catalyst promotes methanation of CO, and thethird catalyst promotes ammonia decomposition. The catalysts can be ineither a pelletized form or a fluidized bed.

Each vessel 28, 30, 32 has an inlet and outlet so that fuel gas is ableto pass through the vessels and contact the respective catalysts. Theinlet of the first vessel 28 is connected to the HGCU system 14 (notshown in FIG. 2), while the outlet of the first vessel 28 is connectedto the inlet of the second vessel 30. The outlet of the second vessel 30is connected to the inlet of the third vessel 32, and the outlet of thethird vessel 32 is connected to the gas turbine 16 via the particulateremoval system 15 (not shown in FIG. 2). Optional coolant injectionports 34, 36 may be located between the first and second vessels 28, 30and between the second and third vessels 30, 32, respectively, toactively control the temperature within each stage and to provide theadded benefit of minimizing potential carbon deposits that can occurover catalyst surfaces in gas environments containing a high CO, lowsteam content.

Several materials are suitable as the catalysts for the presentinvention. Materials which can be used as water-gas-shift catalystsinclude sulfided cobalt-molybdenum and iron-based catalysts such as zincferrite (ZnFe₂ O₄), ferric oxide (Fe₂ O₃), magnetite (Fe₃ O₄), andiron/chromia (90-95% Fe₂ O₃ and 5-10% Cr₂ O₃). Suitable CO methanationcatalysts include cobalt-chromium oxide and an iridium promoted nickelcatalyst commercially available under the tradename G65* from UnitedCatalysts, Inc./SRI International. A nickel based catalyst commerciallyavailable under the tradename HTSR-1 from Haldor Topsoe serves well asthe ammonia decomposition catalyst in the present invention. Thiscatalyst is mechanically stable at 1400° F. and impervious to sulfurpoisoning.

In operation, the gasification unit 12 produces fuel gas in thetemperature range of about 800-1100° F. In one example, the fuel gas isat approximately 925° F. and 30 atmospheres. The composition of thisfuel gas is given in Table 1. This is merely an exemplary compositionbased on the

                  TABLE 1    ______________________________________           Species                 Volume    ______________________________________           CO    30-40%           H.sub.2                 25-30%           CH.sub.4                 ˜0.1%           N.sub.2                 4-5%           CO.sub.2                 10-15%           H.sub.2 O                 15-20%           Ar    ˜1%           NH.sub.3                 500-2500 ppm           H.sub.2 S                  5000-10,000 ppm    ______________________________________

typical output from an oxygen-blown gasifier; other gas compositions arepossible. The fuel gas is then directed through the HGCU system 14 whichreduces the H₂ S content to about 30 ppm. Next, the valves 22 are set todirect the fuel gas through the desired one of the catalytic reactors18, 20.

The fuel gas passes through the first vessel 28 of the selected reactorwhere the first catalyst promotes the water-gas-shift reaction. Thisexothermic reaction raises the temperature of the fuel gas to about1000-1100° F. Poisoning of the first catalyst does not occur,particularly for an iron-based catalyst, because the H₂ S concentrationof the desulfurized gas is below the equilibrium vapor pressure of H₂ Sover iron. The fuel gas then passes through the second vessel 30. At theraised temperature of about 1000-1100° F., the second catalyst is activetoward CO methanation, a reaction that further raises the gastemperature to about 1400° F. Furthermore, the CO methanation reactionsignificantly lowers the hydrogen content of the fuel gas. Coolant isinjected as needed through the coolant injection ports 34, 36 toactively control the temperature within each stage.

Lastly, the fuel gas passes through the third vessel 32. At 1400° F.,the catalyst in the third vessel 32 actively promotes ammoniadecomposition so as to reduce the ammonia content, which is the ultimateobjective of the staged catalytic process. And because the first twostages raised the gas temperature and lowered the hydrogen content, alower final ammonia content is obtained than would have been obtainedthrough ammonia decomposition alone. The fuel gas exits the third vessel32 at approximately 1500-1550° F. and 30 atmospheres. The composition ofthe fuel gas after the staged catalytic process is given in Table 2. Ascan be seen, the ammonia content

                  TABLE 2    ______________________________________           Species                 Volume    ______________________________________           CO    25-30%           H.sub.2                 20-25%           CH.sub.4                 ˜5%           N.sub.2                 5-6%           CO.sub.2                 20-25%           H.sub.2 O                 15-20%           Ar    ˜1%           NH.sub.3                 170-230 ppm           H.sub.2 S                 ˜30 ppm    ______________________________________

is significantly decreased to about 170-230 ppm. The fuel gas issubjected to a final particulate clean up in the removal system 15. Acoolant such as water or nitrogen is then injected through the coolantinjection port 24 to quench the fuel gas, reducing the temperature toabout 1000-1100° F., which is suitable for gas turbine operation. Thistemperature reduction can alternatively be accomplished with a heatexchanger. The fuel gas is then burned and expanded through the gasturbine 16 to generate power. Because of the low ammonia content,combustion of the fuel gas produces minimal NO_(x) emissions.

Alternatively, the first two stages of the FIG. 2 embodiment could becombined into a single stage using a catalyst which promotes bothwater-gas-shift and methanation of CO. FIG. 3 shows such a two stagereactor 118 comprising a first vessel 128 containing a first catalystand a second vessel 130 containing a second catalyst. The first catalystpromotes water-gas-shift and methanation of CO, and the second catalystpromotes ammonia decomposition. Each vessel 128, 130 has an inlet andoutlet so that fuel gas is able to pass through the vessels and contactthe respective catalysts. The inlet of the first vessel 128 is connectedto the HGCU system 14 (not shown in FIG. 3), and the outlet of the firstvessel 128 is connected to the inlet of the second vessel 130. Theoutlet of the second vessel 130 is connected to the gas turbine 16. Acoolant injection port 124 is provided between the catalytic reactor 118and the gas turbine 16 via the particulate removal system 15 (not shownin FIG. 3). An optional coolant injection port 134 may be locatedbetween the first and second vessels 128, 130 to actively control thetemperature within each stage and to minimize carbon deposits that canoccur over the catalyst surfaces.

The embodiment of FIG. 3 operates in essentially the same manner as theFIG. 2 embodiment except that water-gas-shift and CO methanationreactions are accomplished in one stage instead of two. Generally, anyof the catalysts described above for use in catalyzing thewater-gas-shift and CO methanation reactions can be used as the catalystwhich promotes both water-gas-shift and methanation of CO. This isbecause most materials that are active for one reaction tend to beactive for the other. The cobalt-chromium oxide catalyst is particularlysuitable for dual water-gas-shift/CO methanation in the presentinvention.

As an alternative to pelletized or fluidized beds of catalytic material,the catalytic reactors can comprise a plurality of low pressure drophoneycomb structures (such as the-honeycomb structure 50 shown in FIG.4) with the catalysts coated thereon. Each successive honeycombstructure would be coated with one of the catalysts; the number ofhoneycomb structures would thus correspond to the desired number ofcatalytic stages. Alternatively, a single honeycomb structure could beused with the catalysts coated thereon at successive locations. As withthe pelletized or fluidized bed embodiments, the honeycomb structurescould be used in either two or three stage arrangements. The use ofhoneycomb structures would provide lower pressure drops, althoughpressure drop is typically not a problem in IGCC systems.

FIGS. 5 and 6 show two additional reactor embodiments which, unlike theadiabatic reactors of FIGS. 2 and 3, are non-adiabatic. In FIG. 5, acatalytic reactor 218 has three stages in the form of a first vessel 228containing a first catalyst, a second vessel 230 containing a secondcatalyst and a third vessel 232 containing a third catalyst. The firstcatalyst promotes water-gas-shift, the second catalyst promotesmethanation of CO, and the third catalyst promotes ammoniadecomposition. Optional coolant injection ports 234, 236 may be locatedbetween the first and second vessels 228, 230 and between the second andthird vessels 230, 232, respectively. To the extent described thus far,the catalytic reactor 218 is the same as the FIG. 2 embodiment.

Catalytic reactor 218 differs in that first and second heat exchangers238, 240 are located in the first and second vessels 228, 230,respectively. The heat exchangers 238, 240 remove heat from the vessels228, 230 to control the temperature within each stage. The heatexchangers 238, 240 can thus operate in place of, or in addition to, thecoolant injection ports 234, 236. The heat exchangers 238, 240 cancomprise cooling tubes through which water is passed. The absorbed heatconverts the water to steam. The steam generated by the heat exchangers238, 240 is injected into the gas stream through the injection port 224to lower the gas temperature to system limits. This downstream steaminjection makes the overall process adiabatic.

FIG. 6 shows a non-adiabatic catalytic reactor 318 which is a variationof the FIG. 5 embodiment. The catalytic reactor 318 also has threestages in the form of a first vessel 328 containing a first catalyst, asecond vessel 330 containing a second catalyst and a third vessel 332containing a third catalyst. First and second heat exchangers 338, 340are located in the first and second vessels 328, 330, respectively, toremove heat from the vessels 328, 330. As before, these heat exchangers338, 340 can be used in place of, or in addition to, coolant injectionports 334, 336 located between the first and second vessels 328, 330 andthe second and third vessels 330, 332, respectively. In this case, steamgenerated by the heat exchangers 338, 340 is used to drive a steamturbine 42 for additional power generation.

Although FIGS. 5 and 6 show the use of heat exchangers in three stagereactors, the heat exchangers are also applicable to two stage reactors.In a two stage reactor, the first vessel (i.e., the vessel provided witha catalyst which promotes water-gas-shift and CO methanation) would beprovided with a heat exchanger. Steam generated by the heat exchangercould be either injected downstream of the reactor or used to drive asteam turbine.

The foregoing has described an apparatus and process for reducing theammonia content of fuel gas in an IGCC power plant, thereby reducing theNO_(x) emissions from the plant. While specific embodiments of thepresent invention have been described, it will be apparent to thoseskilled in the art that various modifications thereto can be madewithout departing from the spirit and scope of the invention as definedin the appended claims.

What is claimed is:
 1. A method for reducing the ammonia content of afuel gas produced by a gasifier, said method comprising the stepsof:first exposing said fuel gas to a catalyst which promoteswater-gas-shift; next exposing said fuel gas to a catalyst whichpromotes methanation of CO; and then exposing said fuel gas to acatalyst which promotes ammonia decomposition.
 2. The method of claim 1wherein said step of exposing said fuel gas to a catalyst which promoteswater-gas-shift includes raising the temperature of said fuel gas toabout 1000-1100° F., and said step of exposing said fuel gas to acatalyst which promotes-methanation of CO includes raising thetemperature of said fuel gas to about 1400° F.
 3. The method of claim 1further comprising the step of quenching said fuel gas after said stepof exposing said fuel gas to a catalyst which promotes ammoniadecomposition.
 4. A method for reducing the ammonia content of a fuelgas produced by a gasifier, said method comprising the steps of:firstexposing said fuel gas to a catalyst which promotes water-gas-shift andmethanation of CO; and then exposing said fuel gas to a catalyst whichpromotes ammonia decomposition.
 5. The method of claim 4 wherein saidstep of exposing said fuel gas to a catalyst which promoteswater-gas-shift and methanation of CO includes raising the temperatureof said fuel gas to about 1400° F.
 6. The method of claim 4 furthercomprising the step of quenching said fuel gas after said step ofexposing said fuel gas to a catalyst which promotes ammoniadecomposition.